Standalone .github/hooks/*.json postToolUse hook never fires (repo-root, non-plugin)

Describe the bug

A postToolUse hook defined in a standalone, repo-root .github/hooks/*.json file (not shipped as part of a plugin) never fires. This isn't a config error being silently dropped in the way #3872 describes -- the debug log shows no trace of the hook file being discovered or read at all, not even a rejection.

This is adjacent to several existing reports but distinct from each:

• #1730 covers sessionStart specifically, not postToolUse.
• #2540 and #3659 cover hooks shipped inside a plugin's hooks.json, not a standalone repo-root .github/hooks/*.json file.
• #3872 covers a mis-cased event key being silently dropped after being read; our config used the correct camelCase postToolUse key per the documented schema, and the loader appears to never reach it at all.
• #4001 is a related but distinct symptom on the separate .claude/settings.json hook-consumption path (wrong shell, missing env var) -- in our own testing, Copilot CLI didn't attempt to run that hook either, rather than running it incorrectly.

Affected version

@github/copilot (npm) 1.0.80, Windows 11. Confirmed reproducing on Linux as well (WSL2 Ubuntu 24.04, 1.0.83) — see update below.

Steps to reproduce the behavior

1. In a git repo, create .github/hooks/validate-dockerfile.json:

   {
     "version": 1,
     "hooks": {
       "postToolUse": [
         { "type": "command", "bash": "path/to/shared-script.sh", "powershell": "path/to/shared-script.ps1", "cwd": ".", "timeoutSec": 30 }
       ]
     }
   }
   

   (shape taken directly from the documented schema at https://docs.github.com/en/copilot/how-tos/copilot-cli/customize-copilot/use-hooks)
2. The referenced script is instrumented to append its real invocation (stdin, argv, cwd, env) to a log file, so firing can be verified directly rather than inferred.
3. Start a non-interactive session (copilot -p --allow-all-tools "edit the Dockerfile to add one line") that edits a file matching a tool call the hook should intercept.
4. The edit succeeds. The log file is never created -- the hook script is never invoked.

Repeated across 4 independent variations on Windows, all with the same result (hook never fires):

• Baseline config as above, no matcher field.
• Same, with --experimental passed at launch.
• Same, with an explicit "matcher": "*" field added (in case an absent matcher means "match nothing" rather than "match everything").
• Same, but the triggering edit made via the Write tool (creating the file) instead of Edit (modifying an existing file), to rule out a tool-specific gap.

~/.copilot/logs/process-*.log for the Windows runs shows a NativeHookPipelineProcessor registered for postToolExecution in general, but contains zero references to validate-dockerfile.json, .github/hooks, or any hook-discovery trace for the test repo -- consistent with the hook file never being read, not merely filtered out after being read (which is what #3872's fix would surface as a debug-level "ignoring unknown event" line; no such line appears here either).

Update (see comment below for full detail): re-tested on Linux (WSL2 Ubuntu 24.04, 1.0.83) with the same log-level rigor as the Windows finding above -- 3 independent runs, all with complete ~/.copilot/logs/process-*.log showing normal session completion, zero hook-discovery trace in any of them. Confirmed not Windows-specific. (A first Linux attempt, noted in this section previously, had been inconclusive due to an incomplete session; that ambiguity is resolved by the re-test.)

Expected behavior

A postToolUse hook in a standalone repo-root .github/hooks/*.json file, using the documented schema, should fire on a matching tool call -- the same way it does for Claude Code's equivalent PostToolUse hook in .claude/settings.json, which was verified firing correctly in the same test session as a control.

Additional context

• Found while verifying whether a single validator script could be shared across a Claude Code PostToolUse hook and a Copilot CLI postToolUse hook, for a cross-tool developer tooling project.
• Confirmed not Windows-specific -- see the Linux re-test note above.
• This report was drafted with AI assistance (Claude Code) based on a live, instrumented reproduction; happy to provide the raw log files or test fixtures if useful for triage.

Start by reproducing with .github/hooks/validate-dockerfile.json and inspect the complete ~/.copilot/logs/process-*.log output. Trace the NativeHookPipelineProcessor and the standalone .github/hooks discovery path, then verify that a matching postToolUse edit invokes the instrumented script and produces a discovery trace.